1. Field of the Invention
The present invention relates to valve seats and particularly to self locking valve seats for use in internal combustion engines.
2. Description of the Prior Art
It has been common knowledge in the art that successful performance of a valve seat insert requires that there be intimate contact with the surrounding engine material so that adequate heat transfer may occur between the insert and the engine.
The problem of heat transfer is also involved with the problem of securely fastening the valve seat insert in the cylinder head of the engine.
As a means to improve the economy of the Otto type internal combustion engine, auto manufacturers have attempted to switch from cast iron to aluminum block and/or aluminum head engines. This switch has accentuated the problems associated with heat transfer and securely fastening valve seats in the cylinder heads.
A number of solutions have been tried in the past with limited success. Valve seats which are pressed into counterbores and then locked in place with various devices have been used. These have not proved entirely satisfactory because of heat transfer problems, resulting from the buildup of corrosion and carbon deposits around the insert and its fastening device.
Self tapping threads have been used on valve seat inserts as in U.S. Pat. No. 3,868,953 to overcome heat transfer and secure fastening problems. While partially solving those problems this method has introduced new problems of alignment and cost of manufacture. Furthermore, replacement of the insert with the self-cutting thread sometimes results in a loose valve seat since the new insert may not fit tightly into the threadways formed by the previously used valve seat insert.
The method currently used for insertion of valve seat inserts is to lower the temperature of a machined insert by immersion in liquid nitrogen. Then heat the cylinder head to expand the machined opening and then force the insert into the opening.
When the insert and cylinder head reach ambient temperature a tight fit results with good heat transfer characteristics. However, this method is expensive since it involves rigid manufacturing tolerances and the use of cryogenic equipment and the handling of liquid nitrogen. There is also a safety hazard involved in handling liquid nitrogen as part of a large production scale operation.
Also, while this cryogenic procedure has proved satisfactory for initial engine production operations, it does not solve the problem of replacing valve seats in the field.